[LayoutNG] Add NGConstraintSpace class definition

Define initial outline of the constraint space API for LayoutNG by means
of the NG[Derived]ConstraintSpace & NGLayoutOpportunityIterator classes.

BUG=591099
R=ikilpatrick@chromium.org

Review-Url: https://codereview.chromium.org/2176343006
Cr-Commit-Position: refs/heads/master@{#409035}
3 files changed